Despite the identification of tumor-associated antigens in various malignancies, the vast majority of the antigens are at best weakly immunogenic. Dendritic cells (DC), which are extremely efficient antigen presenting cells uniquely capable of sensitizing naive T cells to antigen, have been used to reverse this immunologic unresponsiveness against different tumors. Armed with tumor-associated antigens, DC are capable of priming tumor specific immune responses in vitro and in vivo, often leading to tumor protection in various animal models. Clinical trials using DC have demonstrated that they can induce T cell and B cell immune responses against tumor associated antigens. Moreover, clinical responses have been observed in some patients, particularly with immunogenic tumors such as lymphoma and melanoma. Recently, we have demonstrated that DC loaded with a peptide derived from carcinoembryonic antigen (CEA) can induce T cell immunity to this antigen. Vaccination of fifteen patients with metastatic colorectal cancer who were treated resulted in clinical responses including two complete responses. Nevertheless, advances making DC-based vaccination more potent will be required if this immunotherapeutic approach is to succeed clinically. The current proposal seeks to address several fundamental issues in DC immunotherapy through preclinical and clinical studies in colorectal cancer patients. While our prior studies have utilized blood derived DC enriched through density gradients, recent studies have demonstrated that blood dendritic cells are comprised of at least 2 major subtypes that may prime immunity differently. We will explore the immunogenicity of the two distinct DC subtypes to induce immunity in vitro and in vivo by purifying them directly from the blood and examining their relative capacity to process and present antigen. Moreover, we will develop techniques to purify the different DC subtypes on a clinical scale and vaccinate patients with these subsets to assess whether the different subtypes in fact induce a qualitatively different immune response in vivo. We will also continue to develop and perform immunologic monitoring in these patients with novel in vitro assays to evaluate the immunologic efficacy of these vaccine strategies. The proposed studies will provide the foundation for a more potent DC targeted cancer immunotherapy for the future. [unreadable] [unreadable] [unreadable]